A soybean contains a large amount of high-quality proteins and has been utilized as an excellent protein source from old times. In particular, soy protein isolate is useful as a food material by virtue of its high protein content and various functional characteristics such as emulsification properties, gelation properties, water holding properties etc., and it is used widely as a material for meat products, fish paste products, daily dishes, bread, confectionery, drinks etc.
The soybean protein is composed of various proteins of high molecular and complicated higher-order structure which are classified into 2S, 7S, 11S and 15S proteins etc. based on e.g. ultracentrifuge sedimentation rates, and these proteins have different characteristics even in physical properties.
For example, the soy protein isolate obtained by acid-precipitating soy milk extracted from de-fatted soybeans with water consists essentially of 7S globulin (mainly .beta.-conglycinin) and 11S globulin (mainly glycinin), and each component has peculiar functional characteristics. However, these components are present in the form of their mixture and thus the inherent functional characteristics of each component cannot sufficiently be utilized in practical use.
Therefore, many attempts have been made to fractionate each component to utilize its inherent functions. For example, there are studies and reports of Wolf et al. and Thanh et al. on experimental fractionation, and there are proposals in Japanese Patent LOP Publication Nos. 56843/1973, 31843/1974, 86149/1976, 124457/1980, 153562/1980, 64755/1981, 132844/1982 and 36345/1983. However, these prior methods are still in the experimental stage and are not suitable for industrial fractionation.
Under these circumstances, it is proposed in Japanese Patent LOP Publication 187755/1986 that soybean protein components can be fractionated in an industrial separation method using pH and temperature regulation in the presence of sulfite etc., but troublesome pH and temperature regulations are essential in this method. There are also many investigations for functional improvements by use of proteolytic hydrolysis with proteases. For example, Japanese Patent Publication No. 24262/1973, Japanese Patent Publication No. 1028/1980, Japanese Patent LOP Publication No. 232341/1987, Japanese Patent Publication No. 14941/1992 etc. are concerned with such improvements, but all these methods are related to functional modifications such as in solubility, non-gelation properties etc. by preceding thermal denaturation of soybean protein for promotion of hydrolysis prior to enzymatic reaction, and there are no attempts at functional modifications by decomposing of only a specific component in soybean protein.
It is often hard for an native form of protein such as soybean protein to undergo decomposition with a hydrolytic enzyme such as protease (S. S. Nielsen et al., J. Agric. Food. Chem., 36, 869 (1988)) and thus protein denaturation by heating, alcohol etc. is common practice prior to proteolytic hydrolysis.
The soy protein isolate is a mixture consisting essentially of 7S globulin (mainly .beta.-conglycinin) and 11S globulin (mainly glycinin) as described above, and it is known that there is a difference between the components in degree of denaturation caused under the same conditions. For example, it is known that 11S globulin is denatured easier at acidic pH than 7S globulin (I. Koshiyama, J. Sci. Fd Agric., 23, 853 (1972)), and also that 7S globulin is denatured by heating at lower temperature than 11S globulin (S. Damodaran, J. Agric, Food Chem., 36, 262 (1988)). In the method for enzymatic decomposition up to now, however, it was not possible to specifically and exclusively decompose a specific component in soybean protein, possibly due to uncontrollable protein denaturation treatments, such as excessive heating, alcohol treatment, etc. prior to enzymatic decomposition.
If it is possible to decompose exclusively specific components in soybean protein, a soybean protein having inherent functional characteristics would be obtained from a mixture of the respective components and such protein could be applied further to various fields of foods.
For production of meat products (meat ham, meat sausage, meat bacon, roast pork, meat fried products (fried pork cutlet, tempura) etc.), particularly for production of ham, there is a method in which a pickle solution containing soy protein and other binding materials (proteinous materials) such as egg white, sodium casein, milk protein, blood protein etc., as well as seasonings such as common salt, sugars etc., spices, binding reinforcing agents such as polymerized phosphate etc., coloring agents such as nitrite etc., emulsion stabilizers such as sodium casein etc., antioxidants such as ascorbate etc., seasonings such as sodium glutamate etc., preservatives such as potassium sorbate etc., sweeteners, etc. (referred to hereinafter as "pickle solution of the present invention" is mixed with or injected into meat to improve feelings when eaten, such as water retention characteristics, lipid inclusion properties, binding properties, hardness and elasticity. In the method of injecting the pickle solution into meat, however, if their contents are increased unnecessarily, their viscosity would also be raised and cause difficult injection of such proteinous materials into meat through an injector, whereas if their contents are lowered unnecessarily, the improvement effects could not be obtained as expected.
By virtue of its excellent improvement effects as described above, soy protein is compounded into the pickle solution, but because soy protein itself has high viscosity, there is a limit to its content. To prevent soy protein from raising viscosity, Japanese Patent LOP Publication No. 328939/1993 proposes use of a hydrolysate of soy protein treated with a protease, and Japanese Patent LOP Publication No. 46799/1994 proposes treatment of soybean protein with a special enzyme, but under the present circumstances there is still demand for soybean proteinous materials which are suitable for food manufacturing, excellent in feeling when eaten, in flavor, appearance etc., and easy to handle due to low viscosity when prepared, e.g. in injecting the pickle solution into meat.
Further, soy protein itself has high viscosity and is difficult to drink, so there is a limit to its content in drinks although soy protein is excellent in nutrients.